Essentially water-free polymerized crystalline colloidal array composites having tunable radiation diffracting properties and process for making

ABSTRACT

The present invention is directed to a composite having tunable radiation diffracting properties which includes a flexible, water-free polymeric matrix and a crystalline colloidal array of particles having a lattice spacing, the array being embedded in the polymeric matrix and the lattice spacing changing responsive to stress applied to the polymeric matrix, thereby causing the radiation diffracting properties to change, wherein the polymeric matrix, the lattice spacing and the radiation diffracting properties all return to their original state essentially immediately upon removal of the stress. The present inventive composite is preferably made by a process, which involves forming a preliminary hydrogel polymerized crystalline colloidal array (PCCA), dehydrating the PCCA, and then forming a final, encapsulating polymeric matrix.

CROSS-REFERENCE TO RELATED APPLICATIONS

Priority is hereby claimed to Provisional U.S. Application Ser. No.60/327,074, which is entitled: “POLYMERIZED CRYSTALLINE COLLOIDAL ARRAYCOMPOSITES HAVING TUNABLE RADIATION DIFFRACTION PROPERTIES” filed Oct.3, 2001.

FIELD OF THE INVENTION

The present invention relates generally to radiation filters based oncrystalline colloidal arrays. More specifically, the present inventionis directed to a tunable radiation filter, which includes a highlyordered crystalline array of particles fixed in an essentiallywater-free matrix and a process for making such.

BACKGROUND OF THE INVENTION

A crystalline colloidal array (CCA) is a three dimensionally orderedlattice of self-assembled monodisperse colloidal particles, typicallyamorphous silica or a polymer latex, dispersed in an aqueous ornon-aqueous medium. At high particle concentrations, long-rangeelectrostatic interactions between particles result in a significantinter-particle repulsion, which yields the adoption of a minimum energycolloidal crystal structure with either body-centered cubic orface-centered cubic symmetry.

Crystalline colloidal arrays can be formed having lattice spacingscomparable to the wavelengths of ultraviolet, visible and infraredradiation. It has long been recognized that an array comparable inperiod to the wavelength of electromagnetic waves can provide an analog,i.e., a “bandgap,” which can act as a filter for a particularwavelength. Bragg diffraction techniques have been used to examine CCAswith a view towards identifying their interparticle spacing, latticeparameters and phase transitions. Because CCAs can be fabricated todiffract electromagnetic radiation, including the visible spectrum, sucharrays have potential applications as optical filters, switches,limiters and sensors. However, the low elastic modulus exhibited by aliquid dispersion results in weak shear, gravitational, electric field,or thermal forces having the propensity to disturb the crystalline orderand is a severe drawback to the practical application of CCAs inphotonic devices.

Recently, approaches to develop robust network matrices have beenpioneered to stabilize both organic and inorganic arrays through an insitu polymerization of a monomer around the ordered arrays.Specifically, colloidal crystals arrays have been stabilized throughencapsulation in hydrogel networks and have been referred to aspolymerized crystalline colloidal arrays (PCCAs). However, the PCCAscontain at least 30 percent by volume of water, resulting in theirfragility and propensity for significant changes in optical performancewith water content.

To overcome the drawbacks of the hydrogel networks CCAs have beenencapsulated in essentially water-free polymeric matrices. However, onemotivation for developing a more robust system was to achieve varyingtypes of tunability, i.e., controllable changes of the CCA latticespacings responsive to specific environmental stimuli. Yet, thewater-free PCCAs that have been formed to date have exhibited limitedtuning capabilities. Specifically, prior art composite films composed ofsilica particles in an acrylate polymeric matrix have exhibited bandstop tuning responsive to mechanical stress, though the diffractionwavelength shifts were limited to about 50 nm or less and the time forthe films to return to the optical characteristics of their unloadedstate after the cessation of stress was from two to four hours.

Accordingly, there exists a need in the art for robust composites whichexhibit radiation diffracting properties, which are tunable to asignificant degree responsive to applied stress and which return totheir initial optical characteristics immediately upon the cessation ofstress.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a composite havingtunable electromagnetic wave diffracting properties, which includes aflexible, essentially water-free polymeric matrix and a crystallinecolloidal array of particles having an initial lattice spacing, thearray embedded in the polymeric matrix, wherein the lattice spacingchanges responsive to environmental stimulation applied to the polymericmatrix, thereby causing the electromagnetic wave diffracting propertiesto change, and wherein the array reverts to the initial lattice spacingessentially immediately upon removal of the environmental stimulation.Preferably, the particles of the crystalline colloidal array are chargedparticles. Preferably the particles of the crystalline colloidal arrayare polymeric. Alternatively, the particles of the crystalline colloidalarray may be inorganic. Preferably, the essentially water-free matrix isan acrylate polymer. It is also preferred that the essentiallywater-free matrix is elastomeric. It is also preferred that theessentially water-free matrix is crosslinked. The electromagnetic wavediffracting properties of the composite will change responsive toenvironmental stimulation including mechanical stimulation, thermalstimulation, electrical stimulation, and chemical stimulation.

The present invention is also directed to a method for making acomposite having tunable electromagnetic wave diffracting properties,which includes the steps of:

-   -   a) allowing colloidal particles to self-assemble into a        crystalline colloidal array in a medium;    -   b) adding at least one hydrogel-forming monomer to the medium        containing the crystalline colloidal array;    -   c) polymerizing the at least one hydrogel-forming monomer to        form a polymerized crystalline colloidal array having a hydrogel        matrix;    -   d) dehydrating the hydrogel matrix of the polymerized        crystalline colloidal array;    -   e) swelling the dehydrated polymerized crystalline colloidal        array with at least one monomer; and    -   f) polymerizing the at least one monomer thereby forming an        essentially water-free polymerized crystalline colloidal array.

Preferably, the colloidal particles are charged. Preferably, thehydrogel matrix is a polyethylene glycol. Alternatively, the hydrogelmatrix may be a polyacrylamide. The at least one monomer employed inswelling the dehydrated crystalline colloidal array may be a liquidmonomer or a solid monomer dissolved in a solvent.

The present invention is also directed to a photonic composite having atunable bandgap which is made by a process including the steps of:

-   -   a) allowing colloidal particles to self-assemble into a        crystalline colloidal array in a medium;    -   b) adding at least one hydrogel-forming monomer to the medium        containing the crystalline colloidal array;    -   c) polymerizing the at least one hydrogel-forming monomer to        form a polymerized crystalline colloidal array having a hydrogel        matrix;    -   d) dehydrating the hydrogel matrix of the polymerized        crystalline colloidal array;    -   e) swelling the dehydrated polymerized crystalline colloidal        array with at least one monomer; and    -   f) polymerizing the at least one monomer thereby forming an        essentially water-free polymerized crystalline colloidal array;    -   wherein the bandgap shifts responsive to environmental        stimulation.        Preferably the hydrogel matrix is a polyethylene glycol.        Alternatively, the hydrogel matrix may be a polyacrylamide.        Preferably, the at least one monomer employed in swelling the        dehydrated polymerized crystalline colloidal array is an        acrylate monomer.

Thus, the present invention is directed to a composite having tunableradiation diffracting properties which includes a flexible, water-freepolymeric matrix and a crystalline colloidal array of particles having alattice spacing, the array being embedded in the polymeric matrix andthe lattice spacing changing responsive to stress applied to thepolymeric matrix, thereby causing the radiation diffracting propertiesto change, wherein the polymeric matrix, the lattice spacing and theradiation diffracting properties all return to their original stateessentially immediately upon removal of the stress.

Preferably, the polymeric matrix is elastomeric. One preferred polymerfor use as the present polymeric matrix is poly(2-methoxyethyl acrylate)although any of a nearly limitless number of polymers having appropriateoptical and mechanical properties may be employed.

Polystyrene particles are preferred for the crystalline colloidal arrayof the present invention although, hereagain, any suitable particles canbe used. Examples of such include polymethylmethacrylate, silicondioxide, aluminum oxide, polytetrafluoroethylene or any other suitablematerials which are generally uniform in size and surface charge.

The CCA spacing may provide for radiation diffracting properties in theultraviolet, visible and/or infrared portion or portions of theelectromagnetic spectrum. Preferably, changes in the lattice spacing ofthe crystalline colloidal array effect changes in the compositeradiation diffracting properties which result in diffraction wavelengthshifts of as much as 55 nm or more, more preferably, as 100 nm or more.The recovery time for returning to the initial lattice spacingconfiguration upon removal of stress, as is evidenced by a return to adiffraction wavelength within 2 nm of the original diffractionwavelength, is less than one minute, preferably less than 10 seconds andmost preferably less than two seconds.

Further, the present invention is directed to a method for making acomposite having tunable radiation diffracting properties which includesthe steps of:

-   -   a) allowing colloidal particles to self-assemble into a        crystalline colloidal array in a medium;    -   b) adding at least one hydrogel-forming monomer to the medium        containing the crystalline colloidal array;    -   c) polymerizing the at least one hydrogel-forming monomer to        form a polymerized crystalline colloidal array having a hydrogel        matrix;    -   d) dehydrating the hydrogel matrix of the polymerized        crystalline colloidal array;    -   e) swelling the dehydrated polymerized crystalline colloidal        array by adding at least one further monomer having an affinity        for the hydrogel; and    -   f) polymerizing the at least one further monomer thereby forming        an essentially water-free polymerized crystalline colloidal        array.

One preferred monomer for use as the hydrogel-forming monomer isethylene glycol. Alternatively, acrylamide may be employed. A preferredmonomer for use as the at least one further monomer for forming anessentially water-free polymerized crystalline colloidal array is2-methoxyethyl acrylate although any of a wide variety of monomerscapable of forming essentially water-free polymers having appropriateoptical and mechanical properties may be employed.

Polystyrene particles are preferred for forming the self-assembledcrystalline colloidal array of the present invention although anysuitable particles can be used. Examples of such includepolymethylmethacrylate, silicon dioxide, aluminum oxide,polytetrafluoroethylene or any other suitable materials which aregenerally uniform in size and surface charge.

Additionally, the present invention is directed to a composite havingtunable radiation diffracting properties made by a process whichincludes the steps of:

-   -   a) allowing colloidal particles to self-assemble into a        crystalline colloidal array in a medium;    -   b) adding at least one hydrogel-forming monomer to the medium        containing the crystalline colloidal array;    -   c) polymerizing the at least one hydrogel-forming monomer to        form a polymerized crystalline colloidal array having a hydrogel        matrix;    -   d) dehydrating the hydrogel matrix of the polymerized        crystalline colloidal array;    -   e) swelling the dehydrated polymerized crystalline colloidal        array by adding at least one further monomer having an affinity        for the hydrogel; and    -   f) polymerizing the at least one further monomer thereby forming        an essentially water-free polymerized crystalline colloidal        array;    -   wherein the essentially water-free polymerized crystalline        colloidal array has a lattice spacing which changes responsive        to stress thereby changing the radiation diffracting properties        and wherein the essentially water-free polymerized crystalline        colloidal array, the lattice spacing and the radiation        diffracting properties return to their original states        essentially immediately upon release of the stress.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of this invention, including the bestmode shown to one of ordinary skill in the art, is set forth in thisspecification. The following Figures illustrate the invention:

FIG. 1 is a schematic representation of the process of the presentinvention; and

FIG. 2 is reflectance spectra of a composite in accordance with thepresent invention prior to, during, and following the application of 145kPa compressive stress.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is to be understood by one of ordinary skill in the art that thepresent discussion is a description of exemplary embodiments only, andis not intended as limiting the broader aspects of the presentinvention, which broader aspects are embodied in the exemplaryconstruction.

The present invention is directed to a composite having tunableradiation diffracting properties, i.e., a tunable photonic bandgap. Inessence a crystalline colloidal array (CCA) of particles is “frozen” inits optimum configuration and embedded in a flexible, water-freepolymeric matrix thus forming a “photonic crystal” which can forbidphoton transport for a certain band of frequencies. The lattice spacingof the CCA changes responsive to stress applied to the polymeric matrix,thereby causing the radiation diffracting properties to change. Thus, byapplying a predetermined amount of stress to the polymeric matrix thecomposite can be tuned from one set of optical properties to another. Asis noted above, similar composites have been made in the past.

However, the present composite exhibits improved optical properties,greatly improved tunability, and most importantly, the ability torecover both mechanically and optically almost immediately upon theremoval of stress. These improvements are achieved by a process forforming the present composite which first locks in the original, optimumconfiguration of the CCA prior to forming the final polymeric matrixabout the colloidal particles.

FIG. 1 schematically illustrates the present method for forming atunable composite in accordance with the present invention. Mostbasically, colloidal particles 10 are allowed to self-assemble into acrystalline colloidal array 12 within a medium 14. In order to initiallystabilize the array, a hydrogel-forming monomer is added to the medium14 and polymerized, as by photoinitiated free radical polymerization,into a hydrogel 16, thereby forming a hydrogel-based polymerizedcrystalline colloidal array (PCCA) 18. Thereafter, the water is removedby evaporation to form dehydrated hydrogel-based PCCA 20. The dehydratedPCCA is then swollen in a liquid monomer which has a strong affinity forthe hydrogel-based matrix. That monomer is then polymerized, preferablyphotopolymerized, into an essentially water-free polymeric matrix 22thereby forming final composite 24.

The colloidal particles 10 can be colloidal polystyrene,polymethylmethacrylate, polybutadiene, polyisoprene, silicon dioxide,aluminum oxide, polytetrafluoroethylene or any other suitable materialswhich are generally uniform in size and surface charge. Colloidalpolystyrene is preferred. The particles are chosen depending upon theoptimum degree of ordering and the resulting lattice spacing desired forthe particular application.

In general, any emulsion system including macro ions could be employedto form the CCA. In one embodiment, a CCA can be formed of a hybrid oftwo or more different particle types. For example, a CCA can be formedof a hybrid of two different types of organic particles or inorganicparticles. Alternatively, a CCA can be formed of a hybrid of bothinorganic and organic particles. In general, the macro ions utilized canbe spherical in shape, though this is not a requirement.

A CCA will usually fall into one of two general categories. For example,a CCA can be a sterically packed array, in which the colloidal particlescan usually be from about 10 nm to about 1 μm in diameter. Specifically,the particles can be from about 50 to about 800 nm in diameter. Morespecifically, the particles can be from about 200 to about 500 nm indiameter. Whatever the chosen diameter of the particles, the particlescan contact each other to form an ordered, packed system.

Alternatively, a CCA can be an electrostatically stabilized system, inwhich the colloidal particles are produced such that they exhibit anegative surface charge. When placed in a solution which is pure andnearly free of ionic species, the repulsive interaction between themacro ions can be significant over distances greater than 1 μm. Whendispersed in a polar medium at high particle concentrations,interactions between the surface charge of the particles, coupled withthe consequent diffuse counterion cloud (known as a double layer effect)can result in the adoption of a minimum energy crystalline colloidalstructure having either a body centered cubic (bcc) or face centeredcubic (fcc) symmetry.

As noted above, these CCA systems can be fabricated to exhibit specificperiodicity analogous to electromagnetic wavelengths. For example, theperiodicity of the array can be analogous to electromagnetic wavelengthsin the infrared, visible, or ultraviolet spectrums. This can result inthe appearance of a bandgap in the spectrum. The refractive index ofthese systems can be further adjusted through the addition of variousadditives. For example, dyes, photochromic dyes, or fluorine can beincorporated into the CCA to “tune” the optical effects of the system.CCAs exhibiting optical bandgap effects can then be employed in avariety of active photonic switching and sensory roles.

The composites of the present invention have preferably been formed fromelectrostatically stabilized CCA systems, although sterically packedCCAs may alternatively be employed. In one embodiment, the CCAs utilizedcan be formed using monodisperse cross-linked polystyrene-basedparticles as the colloidal particles, though this is not required forpractice of the invention. These particles can be prepared by usingstandard emulsion polymerization procedures, which are known in the art.

Specifically, an emulsion polymer colloid can be prepared by mixing thedesired monomer with a cross-linking agent, a surfactant to aid in theformation of the emulsion, a buffer to keep the pH of the solutionconstant and to prevent particle coagulation, and a free-radicalinitiator to initiate polymerization. In one embodiment the monomer isstyrene, the cross-linking agent is divinylbenzene, the surfactant issodium lauryl sulfate, the initiator is potassium persulfate and,optionally, an ionic comonomer is also added, such as 1-sodium,1-allyloxy-2-hydroxypropane sulfonate. Other suitable compounds can alsobe used to prepare the emulsion polymer colloid, so long ascompatibility problems do not arise. The particles should then bepurified by the use of centrifugation, dialysis and/or an ion exchangeresin. Purification of the commercially available particles is alsorequired.

Following polymerization, the particles may be stored in an ion exchangeresin. The ion exchange resin should preferably be cleaned prior to use.

The colloidal particles employed can be of any suitable particle size,but in general will be between about 10 nanometers to about 10 micronsin diameter. Specifically, the particles can be between about 20 andabout 500 nanometers in diameter. More specifically, the particles canbe between about 100 and about 200 nanometers in diameter.

In one possible embodiment, the resulting latex produced by the emulsionpolymerization procedures can be dialyzed against deionized water andthen shaken with an excess of mixed bed ion-exchange resin to removeexcess electrolyte. The CCA can then be allowed to self-assemble.

The electrically charged particles are then allowed to self assemble toform a crystalline colloidal array. This assembly takes place in asuitable medium 14, preferably water.

The diffraction characteristics of CCA systems are most accuratelypredicted through the application of dynamic diffraction theory, thoughBragg's law is a reasonable approximation. Of importance to the presentinvention, a CCA can be “tuned” to exhibit some desired periodicity andexhibit a specific bandgap based on the interplaner spacing of thediffracting lattice planes. Interplaner spacing in turn can be afunction of the concentration of colloidal particles forming the CCA. Inother words, the concentration of colloidal particles can be designed oraltered in order that the CCA exhibit a specific bandgap.

Conversely, a shift in the observed bandgap of the system can beevidence of a shift in the interplaner spacing, d, of the orderedsystem. Such a shift in the ordered lattice structure may beattributable to some specific stimulation of the system. For example,when a CCA is formed in a deionized water system, the CCA can opalesceat a certain color due to the optical bandgap effect. If water or someother compound in the system is allowed to escape, due to, for example,evaporation, the observed bandgap can shift due to the increasedconcentration of the colloidal particles and the decreased interplanerspacing of the array. As such, the system will opalesce at a bluer huedue to the change in particle concentration.

Similarly, the addition of a compound to the system can cause a decreasein the concentration of colloidal particles and a relative increase inthe interplaner spacing of the array, thus a red shift in the opticalbandgap can be seen. As a result, such CCAs can be useful in variousoptical switching and sensing technologies.

Following formation of the CCA, the hydrogel monomer is added andpolymerized to form an encapsulated hydrogel polymerized crystallinecolloidal array (PCCA). The PCCA can be formed by any suitable method.In general, such methods can be thin film formation methods. This couldinclude lithography methods, such as, for example, photolithography,various forms of near-field optical lithography, and soft lithography.Alternatively, other forms of thin film formation could be utilized suchas surface templating, layer-by-layer assembly methods, pulsed laserdeposition methods, or through polymerization of the CCA/monomer blendsolution within a defined area. For example, in one embodiment, theCCA/monomer solution can be an aqueous solution including aphotoinitiator injected between two quartz plates separated by aParafilm spacer and then polymerized into a PCCA hydrogel throughexposure to an ultraviolet electromagnetic radiation source for asuitable period of time. In general, no matter what method of productionis used, the product PCCA can have a size defined by the desired finalapplication of the film. For example, the PCCA film can be from about 1to about 1500 μm thick and have length and width dimensions as required.

To provide for more efficient polymerization of the monomer, apolymerization initiator can be added to the CCA/monomer blend. Forexample, in one embodiment, the polymerization process can be aphotopolymerization process. Photopolymerization, though not required,has proven effective due to the limitation of possible disturbing forceswhich could disrupt the ordered system. In this particular embodiment, aphotoinitiator can be added to the CCA/monomer blend. Any suitablephotoinitiator can be used such as, for example, benzoin methyl ether(BME) or 2,2′-diethoxyacetophenone (DEAP). Usually, only a small amountof a photoinitiator is necessary for polymerization of the monomer tooccur. For example, ratios of photoinitiator to monomer can be fromabout 1:100 to about 5:100 to effect polymerization as desired.

Upon polymerization, the hydrogel polymer can form either athermoplastic or a thermoset network, as desired, around the orderedcolloidal particles. If a thermoset polymerized system is desired, acrosslinking agent may be added to the CCA/monomer blended system priorto polymerization. In general, a crosslinking agent can be added to themonomer in a ratio of from about 1:5 to about 1:20 (crosslinking agentto monomer). More specifically, the ratio of crosslinking agent tomonomer can be from about 1:8 to about 1:15.

Any suitable crosslinking agent can be utilized. In general, in anembodiment involving a polyethylene glycol (PEG)-based hydrogel PCCA,the crosslinking agent can also be a PEG-based agent, though this is nota requirement. A non-exhaustive list of possible crosslinking agents caninclude, but is not limited to: poly(ethylene glycol) dimethacrylate,poly(ethylene glycol) diacrylate, poly(ethylene glycol) divinyl ether,poly(ethylene glycol) dioleate, and N,N′ methylene bis acrylamide.

Thus, preferably, a hydrogel-forming monomer is then added to thecrystalline colloidal array medium, along with a cross-linker and aphotoinitiator. Monomers of poly(acrylamide) and its derivatives arewell known as hydrogel-formers and may be employed in accordance withthe present invention. However, poly(ethylene glycol) (PEG) hydrogelsare preferred in accordance with the present invention as it has beendetermined that networks based on PEG may provide more versatile tuningproperties. Of course, a variety of other hydrogel homopolymers andcopolymers may also be advantageously employed in accordance with thepresent invention depending on compatibility with the subsequent, finalmatrix chemistry. In the preferred embodiment, however, thehydrogel-forming monomer is a monomer of poly(ethylene glycol)methacrylate, the cross-linker is poly(ethylene glycol) dimethacrylate,and the photoinitiator is 2,2-diethoxyacetophenone. Photopolymerization,accomplished by exposure to a UV source in the presence of aphotoinitiator, is the preferred means for forming the present hydrogel.

Following its formation, the hydrogel matrix containing the embeddedcrystalline colloidal array is completely dehydrated to form dehydratedpolymerized crystalline colloidal array 20. Preferably, the PCCA isallowed to air dry for a period of days and then placed in a vacuum ovenin order to ensure complete dehydration.

Following dehydration, the dehydrated PCCA is then swollen with a liquidmonomer or monomers that will eventually form the matrix 22 of finalcomposite 24. Any of a wide variety of polymers may be employed formatrix 22 depending on the properties desired for the overall composite.In fact, it has been determined in accordance with the present inventionthat at least the thermal and mechanical properties of the finalcomposite are determined almost exclusively by the final matrix polymeremployed and are not affected significantly by the compositions of thecolloidal particles or the hydrogel. However, certain considerationsmust be taken into account in choosing an appropriate matrix polymer.

Primarily, the monomer or monomers which will eventually form the finalmatrix polymer must have an affinity for the hydrogel. Basically, thisaffinity may be viewed as an indication of mutual solubility such that amonomer which has an affinity for a given hydrogel will readily swellthe dehydrated hydrogel PCCA like water swelling a dried sponge. Amonomer which does not have an affinity for the particular hydrogelwhich has been employed will be repelled by the dehydrated hydrogel PCCAand will not swell it. Of course, this required affinity can beanticipated and employed as a determining factor in choosing anappropriate hydrogel which is compatible to a desired final matrixpolymer. Thus, for a PEG hydrogel, a preferred monomer is a PEGfunctionalized acrylate such as 2-methoxyethyl acrylate.

Also of primary importance is that the monomer or monomers employed mustbe a precursor or precursors to an essentially water-free matrix. Thatis, when polymerized the monomer or monomers must form a homopolymer orcopolymer which is not a hydrogel. The formation of an essentiallywater-free, non-hydrogel, polymeric matrix is key to achieving therobust, readily tunable composite of the present invention.

Secondly, it is preferred that the monomer which swells the dehydratedhydrogel PCCA and eventually forms the final matrix polymer is a liquid.However, a solid monomer dissolved in an appropriate solvent may also beemployed.

Finally, depending on the particular end-use application, it may bedesirable for the final composite to possess certain optical, thermaland/or mechanical properties. As was noted above, these may all betailored by choosing an appropriate matrix polymer. For example,although a preferred monomer for forming the final matrix polymer is2-methoxyethyl acrylate, it has been found in accordance with thepresent invention that the glass transition temperature of the finalcomposite can be altered by copolymerization of additional acrylatemonomers or through a complete substitution. For example, the glasstransition temperature of the composite made in accordance with Example2, below, is about −35° C. The glass transition temperature of thecomposite made in accordance with Example 3, below, is in the range of40–50° C. The composite made in accordance with Example 4, below, has acopolymer matrix wherein the dehydrated hydrogel PCCA has been swollenwith a 50/50 blend of 2-methoxyethyl acrylate and 2-methoxyethylmethacrylate, the two monomers employed in Examples 2 and 3,respectively. Thus, the composite of Example 4 has a glass transitiontemperature between those other two composites. Thus, in one preferredembodiment, 2-methoxyethyl acrylate including 1% by weight of ethyleneglycol dimethacrylate is employed to swell the dehydrated PEG-basedhydrogel PCCA. A photoinitiator such as 2,2-diethoxyacetophenone isadded. Subsequent photopolymerization yields a water-free, robustcomposite.

Preferably, the polymer chosen for use as the final composite matrixmaterial is sufficiently flexible, most preferably elastomeric, in orderto provide a final composite which is capable of exhibitingmechanochromic properties. That is, deformation of the matrix by appliedstress causes deformation of the embedded crystalline colloidal arraylattice spacing and, therefore, a change in the radiation diffractingproperties of the composite. However, it should be noted that presentinventive composite may be tailored to exhibit tunability responsive toother environmental stimuli such as, for example, changes intemperature, exposure to certain chemicals, or exposure to electric orelectromagnetic fields.

A particular advantage of the present invention, however, is the abilityto provide PCCA composites which are tunable responsive to appliedstress and, most particularly, which are capable of returning to theoriginal unstressed state, both physically and optically, within secondsof removal of the applied stress. That is, upon removal of appliedstress composites in accordance with the present invention return to adiffracted bandwidth within 1 to 2 nanometers of the original unstressedbandwidth essentially immediately.

Reference now will be made to possible embodiments of the invention, oneor more examples of which are set forth below. Each example is providedby way of explanation of the invention, not as a limitation of theinvention. In fact, it will be apparent to those skilled in the art thatvarious modifications and variations can be made in this inventionwithout departing from the scope or spirit of the invention. Forinstance, features illustrated or described as part of one embodimentcan be used on another embodiment to yield a still further embodiment.Thus, it is intended that the present invention cover such modificationsand variations as come within the scope of the appended claims and theirequivalents.

EXAMPLE 1

Monodisperse crosslinked polystyrene particles were prepared using anemulsion polymerization procedure which involved mixing styrene monomerwith divinylbenzene, a cross-linking agent, sodium lauryl sulfate, asurfactant, and potassium persulfate, a free-radical initiator forinitiating polymerization. The resulting particles were dialyzed againstdeionized water and then shaken with an excess of mixed bed ion-exchangeresin. After cleaning, the particle diameter was measured to be 109±26nm. The cleaned suspension with then diluted with deionized water untilan angle dependent iridescence was observed. Drying a known mass of thesuspension in an oven at 80° C. overnight then under vacuum for two daysresulted in a calculated particle density of 2.6×10¹⁴ cm⁻³.

The crystalline colloidal arrays which formed in the deionized waterwere encapsulated in a hydrogel matrix prepared by an in situphotopolymerization procedure. The matrix materials included a monomerof poly(ethylene glycol) methacrylate (PEGMA, M_(n)=360), a crosslinkerof poly(ethylene glycol) dimethacrylate (PEGDMA, M_(n)=550), and aphotoinitiator of 2,2-diethoxyacetophenone (DEAP). The PEGMA and PEGDMAwere stored in Nalgene® (Nalge Nunc Int'l Corp., Rochester. N.Y.)plastic laboratory containers over a mixed bed ion-exchange resin for atleast 48 hours prior to their use, while all other matrix materials wereused as-received.

The procedure for generating a hydrogel polymerized crystallinecolloidal array film included combining all the components of the PCCAin a Nalgene® container and allowing the mixture to shake with an excessof a mixed bed ion-exchange resin for at least two hours prior toinjecting the mixture between two quartz plates separated by a 500 μmparafilm spacer. The film was then polymerized by exposing the assemblyto a UV source for four minutes.

EXAMPLE 2

A hydrogel PCCA film made in accordance with Example 1 was removed fromthe quartz plates and allowed to air dry for two days and then placed ina vacuum oven at 35° C. The resulting clear film was then swollen in asolution of 2-methoxyethyl acrylate for two days. To this solution,ethylene glycol dimethacrylate and DEAP was added and the formulationwas crosslinked by a twenty-minute exposure to a UV lamp. All chemicalswere purchased from either Aldrich or Acros Organics.

FIG. 2 is a reflectance spectra of the composite of the present Examplecollected on an Ocean Optics PC2000 fiber optic spectrometer taken atnormal incidence in an initial stress-free state, under a compressiveloading, and upon removal of the applied stress. In the initialstress-free state, the position of the band stop is at 610 nm. Uponapplying a 145 kPa compressive stress, the band stop shifts down to awavelength of 517 nm, a 93 nm variation. Additional compressive stressresulted in increasingly larger band stop shifts, with shifts of 120 nmbeing attainable. However, with increasing stress, the peaks becamebroader and less well defined due to the introduction of disorder in thearray. As is shown in FIG. 2, removal of the compressive stress resultsin the immediate return of the band stop position within 1–2 nm of theoriginal stress-free state. It was found that repeated straining of acomposite in accordance with the present invention did not result in anypermanent change in the observed optical characteristics unless the filmwas mechanically degraded.

EXAMPLE 3

A hydrogel PCCA film made in accordance with Example 1 was removed fromthe quartz plates and allowed to air dry for two days and then placed ina vacuum oven at 35° C. The resulting clear film was then swollen in asolution of 2-methoxyethyl methacrylate for two days. To this solution,ethylene glycol dimethacrylate and DEAP was added and the formulationwas crosslinked by a twenty-minute exposure to a UV lamp. All chemicalswere purchased from either Aldrich or Acros Organics.

EXAMPLE 4

A hydrogel PCCA film made in accordance with Example 1 was removed fromthe quartz plates and allowed to air dry for two days and then placed ina vacuum oven at 35° C. The resulting clear film was then swollen in a50/50 solution of 2-methoxyethyl acrylate and 2-methoxyethylmethacrylate for two days. To this solution, ethylene glycoldimethacrylate and DEAP was added and the formulation was crosslinked bya twenty-minute exposure to a UV lamp. All chemicals were purchased fromeither Aldrich or Acros Organics.

These and other modifications and variations to the present inventionmay be practiced by those of ordinary skill in the art, withoutdeparting from the spirit and scope of the present invention, which ismore particularly set forth in the appended claims. In addition, itshould be understood that aspects and various embodiments may beinterchanged either in whole or in part. Furthermore, those of ordinaryskill in the art will appreciate that the foregoing description is byway of example only, and is not intended to limit the invention sofurther described in such appended claims.

1. A method for making a composite having tunable electromagnetic wavediffracting properties comprising the steps of: a) allowing colloidalparticles to self-assemble into a crystalline colloidal array in amedium; b) adding at least one hydrogel-forming monomer to the mediumcontaining the crystalline colloidal array; c) polymerizing the at leastone hydrogel-forming monomer to form a polymerized crystalline colloidalarray having a hydrogel matrix; d) dehydrating the hydrogel matrix ofthe polymerized crystalline colloidal array; e) swelling the dehydratedpolymerized crystalline colloidal array with at least one monomer; andf) polymerizing the at least one monomer thereby forming an essentiallywater-free, non-hydrogel polymerized crystalline colloidal array.
 2. Themethod set forth in claim 1 wherein said colloidal particles arecharged.
 3. The method set forth in claim 1 wherein the hydrogel matrixcomprises a polyethylene glycol.
 4. The method set forth in claim 1wherein the hydrogel matrix comprises a polyacrylamide.
 5. The methodset forth in claim 1 wherein the step of swelling the dehydratedpolymerized crystalline colloidal array with at least one monomercomprises swelling the dehydrated polymerized crystalline colloidalarray with a liquid monomer.
 6. The method set forth in claim 1 whereinthe step of swelling the dehydrated polymerized crystalline colloidalarray with at least one monomer comprises swelling the dehydratedpolymerized crystalline colloidal array with a solid monomer dissolvedin a solvent.
 7. A photonic composite having a tunable bandgap made by aprocess comprising the steps of: a) allowing colloidal particles toself-assemble into a crystalline colloidal array in a medium; b) addingat least one hydrogel-forming monomer to the medium containing thecrystalline colloidal array; c) polymerizing the at least onehydrogel-forming monomer to form a polymerized crystalline colloidalarray having a hydrogel matrix; d) dehydrating the hydrogel matrix ofthe polymerized crystalline colloidal array; e) swelling the dehydratedpolymerized crystalline colloidal array with at least one monomer; andf) polymerizing the at least one monomer thereby forming an essentiallywater-free polymerized crystalline colloidal array; wherein the bandgapshifts responsive to environmental stimulation.
 8. The photoniccomposite set forth in claim 7 wherein the hydrogel matrix comprises apolyethylene glycol.
 9. The photonic composite set forth in claim 7wherein the hydrogel matrix comprises a polyacrylamide.
 10. The photoniccomposite set forth in claim 7 wherein the at least one monomer employedin swelling the dehydrated polymerized crystalline colloidal arraycomprises an acrylate monomer.